This section introduces aspects that may facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Wireless communication systems are advancing to provide good service quality, support a high data rate and keep up with the continuously increasing demand for wireless data traffic.
Multi-carrier system is a technique available for increasing the bandwidth available for communication by employing simultaneously more than one carrier for communication by a single communication device. A multi-carrier system uses more than one carrier in the downlink and/or the uplink.
For bandwidth extension, carrier aggregation (CA) was introduced to the LTE-A system and it was firstly defined in 3GPP Rel-10 and by which two or more component carriers are aggregated to achieve a wider-band transmission. An LTE-A specification supports carrier aggregation for both continuous and non-continuous component carriers. The carrier aggregation increases bandwidth flexibility by aggregating the non-continuous component carriers. A component carrier is used as an UL component carrier or a downlink (DL) component carrier. Now CA is an important feature in LTE which can improve both system capacity and User Equipment (UE) performance. When the UE is configured with the carrier aggregation, the UE is allowed to receive and transmit data on one or multiple component carriers to increase the data rate. In the LTE-A system, it is possible for the eNB to configure the UE different numbers of UL and DL component carriers which depend on UL and DL aggregation capabilities, respectively.
Moreover, the component carriers configured to the UE necessarily consists of one DL primary component carrier (PCC) and one UL primary component carrier. Component carriers other than the primary component carriers are named UL or DL secondary component carriers (SCCs). The numbers of UL and DL secondary component carriers are arbitrary, and are related to the UE capability and available radio resource. One of the serving cells is designated as a “primary cell”. A serving cell that is not a primary cell is referred to as a secondary cell. The functions provided by the primary cell include being responsible for many control plane related events, e.g. providing the necessary information for NAS, e.g. TAI, providing input for security, determining RLF, being the reference Carrier to determine handover, being the carrier to carry PUCCH channel etc.
When the communication link between the UE and the primary cell on the PCC is interrupted, the communication between the UE and the network fails even if the communication link between the UE and a secondary cell is sustained. Given the importance of the link between the UE and the primary cell considering the functions provided by the primary cell, in order to minimize problems associated with radio link quality and other issues, it is necessary to ensure that the primary cell is the best cell of the serving cells in terms of quality. Such a requirement then makes it necessary to change the primary cell when the current primary cell is no longer the best cell in radio link quality. According to 3GPP Rel-13, primary cell can only be changed with handover procedure (i.e. with security key change and RACH procedure).